As reported, for example, in Prota, Progress in the Chemistry of Melanins and Related Metabolites, Med. Res. Reviews, 8:525-56 (1988), melanins are naturally occurring pigments present in hair and skin. In humans, biosynthesis takes place in tyrosinase containing melanocytes. The tyrosinase enzyme catalyzes the hydroxylation of tyrosine to dopa and its subsequent oxidation to dopachrome. Once formed, dopachrome undergoes a series of complex reactions in the formation of eumelanins and phaeomelanins.
Melanins provide black and deep brown pigments, and are formed by oxidative polymerization of 5,6-dihydroxyindole derived biogenetically during the melanogenesis. On the other hand, phaeomelanins provide yellow to reddish brown pigmentation to hair and are formed by oxidative polymerization of cystein-S-yl-dopas via 1,4-benzothiazine intermediates.
Synthetic 5,6-dihydroxyindole (DHI) has been disclosed in the prior art for use in hair and skin dyeing. For example, U.S. Pat. No. 2,934,396 to Charle discloses a process for dyeing hair by contacting hair with an aqueous solution of DHI having a pH of at most 7 for 5 to 60 minutes, followed by an application of an aqueous solution capable of inducing oxidation and/or polymerization of DHI.
Dopa and dopamine are disclosed as hair dyeing precursors in the process of Herlihy, U.S. Pat. No. 4,746,322, wherein the aqueous hair dyeing composition comprises said precursor, an organic compound to assist dye dispersion and an iodate or periodate. The dopa or dopamine dye precursor is present in the aqueous hair dye composition in an amount of from about 1 to about 100 mg/ml, preferably from about 5 to about 25 mg/ml. Dopamine is preferred, according to Herlihy. The iodate or periodate is present in the composition at a concentration of 1 to about 50 mg/ml, while the dispersing agent is present in an amount of from about 0.1 to 30% (wt./vol.). Optionally, a color modifier can be incorporated into the aqueous composition of Herlihy, at a level of from about 0.1 to about 10 mg/ml. pH may be maintained between about 3 to about 7 by incorporation of an effective amount of a buffer. According to Herlihy, the above-described aqueous compositions disperse the dye on the hair shaft "with little or no penetration into the hair shaft." Column 2, lines 56-58.
The prior art fails to provide a commercially feasible process for effectively, permanently dyeing hair using dopa as a starting reagent. It is believed this failing is attributable to an inability of the prior art processes in making a melanin precursor available on the hair at concentrations suitable for its diffusion into the hair, for subsequent conversion to nondiffusable melanin, as further explained in detail below.
Indeed, the inability to provide an inexpensive yet effective process for dyeing hair with a melanin precursor has prevented use of melanogenesis in the commercial dyeing of hair.
Interest in melanogenesis to dye hair is quite high, however. This is because synthetic melanin pigments provide an exceptionally natural-looking deep brown or black color. Moreover, they are not irritating to the skin. Nor are they mutagenic.
It has now been found, quite surprisingly, that an aqueous hair dyeing process wherein an effective melanin-forming hair dyeing amount of 5,6-dihydroxyindole is generated during the reaction of dopa with an oxidant can be practiced inexpensively and under commercially feasible conditions, to achieve a permanent hair color.
It has also been found that an aqueous hair dyeing process, wherein an effective melanin-forming hair dyeing amount of a nitrogenous phenolic, especially indolic, melanin precursor is generated during the reaction of select substituted dopa compounds with an oxidant, can be practiced inexpensively and under commercially feasible conditions, to achieve a permanent hair color. Advantageously, the utilization of the substituted dopa compounds of the present invention is conducive to the attainment of a range of hair color shades, in contrast to the use of dopa alone as the starting reagent, which is capable merely of providing gray or black pigmentation to hair.
It has additionally been found that even further and more desirable color modifications to hair dyed in accordance with the process of the present invention may be obtained by including in the reaction mixture, along with the dopa and/or substituted dopa compound, one or more hair dye compounds selected from the group consisting of direct dyes, primary intermediates and couplers.
The hair dyeing process of the present invention contemplates the preparation of an aqueous hair dyeing composition by reacting dopa or a selected substituted dopa compound as hereinafter defined, with a ferricyanide oxidant to form a melanin-forming hair dye precursor, and applying the aqueous composition to the hair. The melanin precursor contained in said aqueous composition is capable of diffusing into the hair shaft in an amount effective to dye hair permanently upon its coincident conversion to melanin while in the hair.
The aqueous hair dyeing composition is produced by initiating reaction between the dopa species or a salt thereof with an inorganic oxidant that is a soluble ammonium, alkali or alkaline earth metal salt, especially sodium and potassium salts, of ferricyanide in an aqueous reaction medium buffered by sufficient buffering agent to maintain the reaction medium pH from about 6 to about 10 throughout the series of reactions that take place leading to the melanin precursor.
In order to achieve the permanent dyeing of hair in accordance with the process of the present invention, it is critical to generate melanin from the melanin-forming hair dye precursor in the aqueous hair dye composition in such amount as to effect a color change to the hair. The total color change may be gradually obtained by several applications of the composition over time, or may be effected by a one-time application of the composition, depending on the concentration of the dopa species, the duration of application, and the desires of the user. It is further critical that the hair dye composition be applied to the hair prior to the substantial formation of melanin so that the melanin precursor formed during the reaction may diffuse into the hair prior to the generation of melanin, the melanin then being formed within the hair. It is additionally important that the process for dyeing hair as described herein be capable of completion within less than about one hour.
In the case in which the dopa species is dopa or a salt thereof, the reaction with the oxidant leads to the formation of 5,6-dihydroxyindole, which melanin precursor, upon its conversion to melanin, provides hair with a permanent black color. In the case of the selected substituted dopa compounds, melanin precursors are obtained which, upon conversion to melanin, produce a range of shades depending upon the selection of the substituted dopa compound.
A further aspect of the invention is the optional incorporation of an hair dye component selected from the group consisting of direct dyes, primary intermediates, couplers and mixtures thereof in the reaction mixture. Following the initial dopa species-oxidant reaction, it is believed that the direct dyes, primary intermediate(s) and/or coupler(s) present in the reaction mixture react at least in part with the intermediate compounds formed prior to the melanin precursor, thereby providing chromatic characteristics to the melanins ultimately obtained.
In another aspect of the present invention, it has been found that the formation of indolic melanin precursors such as 5,6-dihydroxyindole is hastened by proper selection and amount of the buffer, apart from its requirement for maintaining pH of the reaction medium. Preferably, the buffer is a phosphate, carbonate or bicarbonate, and typically is included in substantial excess over the amount needed to maintain the requisite pH.
In yet another aspect of the present invention, the process for dyeing hair contemplates treatment of the hair with agent(s) that promote melanin formation, e.g., a solution of a metal ion salt, which treatment accelerates the formation of melanin from the ultimate indolic precursor. Treatment with the promoting agent may be a pre- or a post-treatment, or in some instances may be conducted simultaneously with the application of the hair dye composition of the present invention.
The process of the present invention may conveniently be practiced by providing premeasured amounts of the reactants in separate containers packaged in kit form. The user simply admixes the reactants on or with subsequent application to the hair and allows the composition while it is reacting to remain on the hair for the prescribed period of time. It is seen that no special expertise is required to carry out the process, and accordingly the product and process are equally suitable for in-home use by the nonprofessional as well as salon use by the professional. Advantageously, the product in kit form is shelf-stable and is therefore suitable for retail sale and without precautions generally required for melanin-forming precursors, such as 5,6-dihydroxyindole, e.g., storage under anaerobic conditions.